The present invention relates to a resin composition for exterior automotive parts, and more particularly to a resin composition having excellent paint coatability and moldability, a good balance of impact resistance, ductility, mechanical strength, thermal deformation resistance, brittle temperature, hardness, etc., small molding shrinkage and linear expansion coefficient, and a high paint coating peeling resistance in high-temperature, high-pressure washing.
Since polypropylene is light and has excellent mechanical strength, it is used in various applications. However, since it is poor in impact resistance, its copolymers such as propylene-ethylene block copolymers are used. Also, proposals have been made to provide a resin composition comprising a propylene-ethylene block copolymer, an ethylene-.alpha.-olefin copolymer elastomer and an inorganic filler such as talc.
Japanese Patent Laid-Open No. 61-12742 discloses a resin composition comprising (a) 62-57 weight % of a propylene-ethylene block copolymer having an ethylene content of 2-3 weight % and a melt flow rate of 40-45 g/10 minutes, (b) 26-28 weight % of an ethylene-propylene copolymer elastomer having an ethylene content of 70-80 weight % and a Mooney viscosity ML.sub.1-4 (100.degree. C.) of 55-58, (c) 2-3 weight % of a high-density polyethylene having a density of 0.955-0.960 g/cm.sup.3 and a melt flow rate of 18-22 g/10 minutes, and (d) 10-12 weight % of talc having an average size of 1.8-2.2 .mu.m and a specific surface area of 36000-42000 cm.sup.2 /g, the resin composition having a melt flow rate of 13-18 g/10 minutes, a density of 0.950-0.980 g/cm.sup.3, a flexural modulus of 11500-14000 kg/cm.sup.2, a linear expansion coefficient of 7.times.10.sup.-5 -10.times.10.sup.-5 cm/cm/.degree.C. in a temperature range between 20.degree. C. and 80.degree. C., and a surface gloss (measured by a 60.degree.-60.degree. method according to JIS Z8741) of 55% or more.
Japanese Patent Laid-Open No. 1-149845 discloses a resin composition comprising (a) 59-74 weight % of a propylene-ethylene block copolymer containing 5-12 weight % of a boiling p-xylene-soluble component having an ethylene content of 20-60 weight %, and having an ethylene content of 1-7 weight % and a melt flow rate of 15-50 g/10 minutes, (b) 35-20 weight % of an ethylene-propylene copolymer elastomer having a propylene content of 20-60 weight % and a Mooney viscosity MLl.sub.1+4 (100.degree. C.) of 100-150, and (c) 3-6 weight % of talc having a specific surface area of 30000 cm.sup.2 /g or more and an average size of 0.5-2.0 .mu.m.
However, both resin compositions of Japanese Patent Laid-Open Nos. 61-12742 and 1-149845 are susceptible to thermal deformation, and bumpers made of such resin compositions would be easily deformed due to the difference in a linear expansion coefficient between the resin compositions and the automotive body parts in a high-temperature environment, resulting in the deterioration of automotive body appearance. Also, since beautiful coatings are recently formed on bumpers, etc., the resin compositions for bumpers, etc. should have good paint coatability in order to provide the automotive bodies with good appearance.
In the above-described resin compositions comprising the propylene-ethylene block copolymer, the ethylene-propylene copolymer elastomer (or high-density polyethylene) and talc, their flowability is controlled for various applications by adjusting ethylene/propylene ratios and molecular weights, etc. in the propylene-ethylene block copolymer and the ethylene-propylene copolymer elastomer. However, the control of such parameters fails to provide resin compositions having well balanced properties such as paint coatability, moldability, rigidity, ductility, thermal deformation resistance, brittle temperature, hardness, etc. on levels suitable for exterior automotive parts.
As a polypropylene resin composition for automotive bumpers having an improved paint coatability, Japanese Patent Laid-Open No. 57-55952 discloses a polypropylene resin composition comprising 55-65 weight % of a crystalline propylene-ethylene block copolymer containing 5-10 weight % of ethylene, a propylene component, 97 weight % or more of which is insoluble is boiling n-heptane, and a component soluble in p-xylene at a room temperature having an intrinsic viscosity (in decalin at 135.degree. C.) of 3-4 and a melt flow rate of 2-10; 30-35 weight % of an amorphous ethylene-propylene copolymer elastomer having an intrinsic viscosity (in decalin at 135.degree. C.) of 2.0-3.5 and a Mooney viscosity ML.sub.1+4 (100.degree. C.) of 40-100; and 5-15 weight % of talc having an average size of 0.5-5 .mu.m. This resin composition shows an improved resistance to peeling of coatings formed thereon, but it fails to show a sufficient peeling resistance of the coatings in washing under severe conditions such as high temperature and/or high pressure.